AbstractThe type section of the Fossil Butte Member of the Green River Formation at the east of Fossil Butte provides a
nearly complete history of Fossil Lake throughout its history. The base of formation rests on the red fluvial Wasatch Formation.
The limestones of the Green River Formation grade laterally into the Wasatch Formation as well. The Green River Formation is
easily divided into three basic units here, each representing a significantly different phase of Fossil Lake.

The lower unit is composed primarily of bioturbated limestones, but grades southward into a well developed lake
deposit with laminated micrites containing abundant fossil fish.

The middle unit is composed of kerogen-rich laminated-micrite that has been extensively quarried for its abundant fossil fish.
Fossil Lake expanded to its maximum size during middle unit time, however it suffered periods of near total evaporation,
as evidenced by beds of dolomicrite. The fossil resources of this deposit are spectacular and unique, but may be threatened
by multiplied commercial quarrying activities over the past few years.

The upper unit is composed largely of dolomicrite containing salt casts of sodium carbonates. Fossil fish are absent from
the upper unit at Fossil Butte. Fossil Lake freshened only briefly for a short period of time before being totally filled in by
the encroaching fluvial systems surrounding the lake.

Much remains to be discovered about this unique lake deposit, and no doubt our view of Fossil Lake will change with
those new discoveries.

Introduction

Although we do not have time machines to visit the past,
we can come pretty close with the modern tools of
geology and paleontology and with the skills to apply them. It
is the unknown and mystery of the past that motivates many
of us to pursue our professions as geologists and paleontologists.
It is like putting the pieces of a puzzle together... the
more pieces that we can fit together, the clearer the picture becomes.
The Eocene lake deposits (Fossil Lake) of Fossil Butte
National Monument and surrounding areas (known in total
as Fossil Basin) provide an opportunity to recreate the
puzzle and interpret the past in nearly complete way. Most of
the puzzle pieces are there because of continuous deposition
without periods of erosion, because of incredible preservation
of life rarely seen elsewhere, and because of the way nature
has
dissected these ancient lake beds into a suite of canyons
and buttes that provide a nearly complete three dimensional
view of Eocene Fossil Lake. We will now take a walk through
time as we walk up through the type section at Fossil Butte
National Monument; we shall imagine standing on the
ancient shore of this unique lake, and we shall observe the
physical lake environment, as well as the life and death of the
myriad of creatures that inhabited the lake and it's surroundings.
This paper will serve as an introduction to more detailed papers
in this volume about Fossil Butte National Monument that
provide in depth views of Fossil Lake in regards to specific
topics and time periods.

The Type Section Overview

Stratigraphic Nomenclature.Figure 1 provides a
stratigraphic overview of the Green River Formation at Fossil Butte.
Figure 1The Green River Formation in Fossil Basin at Fossil Butte is divided into three units as indicated. Note the marker beds to
the right of the section that are referred to in the text. XRD mineralogy provides a view of the relative amounts of calcite, dolomite, and
total silicate minerals (quartz, feldspar, zeolites, clays). Fossil fish occur in units indicated with solid bars.

The basic lacustrine stratigraphy was first described and
divided into the Fossil Butte and Angelo Members by Oriel
and Tracey (1970) and later studied in more detail by
Buchheim (1994a and 1994b) and Buchheim and Eugster (1998).
Buchheim (1994a) informally divided the Fossil Butte
and Angelo Members of the Green River Formation up into
the lower, middle, and upper units. This was necessary in
order to accurately incorporate new knowledge obtained about
the stratigraphy including the discovery of a thick sequence
of fluvial-lacustrine rocks best developed in the Little
Muddy Creek and more southern canyons and described in detail
by Biaggi (1989). In addition, Buchheim (1994a) found
that Fossil Lake underwent three phases: a shallow freshwater
phase (lower unit); a relatively deeper freshwater to saline
phase (middle unit) and a mostly hyper saline-alkaline phase
(upper unit). The boundaries of these units can be accurately
located at most locations and are identified by mapable marker beds.

Previous stratigraphic and paleontologic
investigations.Cope (1877 and 1884). Peale (1879, p. 535)
first described the geology and fossil fishes of the Green River
Formation. The geologic history with emphasis on
paleontology was summarized by McGrew and Casilliano (1975). A
detailed description of the paleontology of the entire Green
River Formation was provided by Grande (1984). Grande
and Buchheim (1994) provided a detailed analysis of lateral
variations in paleontology and sedimentology in Fossil Lake.
A detailed summary of the Green River Formation of Fossil
Basin was completed recently by Buchheim and Eugster (1998).

Recent Investigators who have made detailed
descriptions of the sedimentology, stratigraphy, and paleontology of
Fossil Lake include: Biaggi (1989), who studied an early phase
of Fossil Lake during which the lower unit was deposited;
Loewen and Buchheim (1997) who reported on the saline
to freshwater transitions in Fossil Lake as revealed in the
upper unit; Cushman (1983) who conducted the only study on
fossil pollen in the lake beds of Fossil Basin; Leggitt and
Buchheim (1997) who reported on a significant mass mortality beds
of fossil birds in the southern part of Fossil Basin (see also
Leggitt, 1996); Trivino (1996) who completed a detailed spatial
analysis of unit 5b, an oil shale bed (kerogen-rich
laminated-micrite) and recreated the paleogeography of Fossil Lake during
one period of Fossil Lake's history.

A Walk Through Time

Lower Unit time.At Fossil Butte one is impressed
with the bold white cliffs of the Green River Formation that
contrast markedly with the underlaying slope-forming and
variegated red and purple Wasatch Formation. The Wasatch
Formation represents a vegetated, fluvial food plain that
was crossed by numerous rivers and streams. If you stand at
the base of the contact between these two formations and
look south and imagine yourself back in early Eocene time,
you would actually be standing on the north shore of a lake
that was well developed in the southern part of Fossil Basin.
However, the lake was usually shallow and was frequently filled
in by fluvial sedimentation. It was surrounded by broad
mud-flats that were mud-cracked. These mudflats were
frequently
swallowed up by a lake that experienced numerous
expansions and contractions. Subsidence in Fossil Basin
eventually exceeded sedimentation and the lake deepened and
flooded the fluvial plain until it stretched north of Fossil Butte
another 30 kilometers.

Fossil Lake never attained depths greater than four
or five meters at Fossil Butte during lower unit time. It
formed a hyper saline lake at least twice and probably totally
desiccated. The two meter-plus thick beds of dolomite are
evidence of this. The massive white limestones of this unit
are heavily bioturbated as indicated by abundant trace fossils
with meniscus fillings indicating sediment-feeding organisms,
such as insect larvae, worms, or shrimp.

As you near the top of the lower unit you observe a
3-4 meter thick sequence of alternating siliciclastic mudstones
and laminated micrites. Close study of the micrites reveal
that they are composed of alternating laminae of micrite and clay.
This unit grades southward into the sandstone tongue of
the Wasatch Formation, that provides a prominent marker
bed throughout the southern half of Fossil Basin. This
sandstone has been interpreted as a prograding delta (Peterson, 1987).
At Fossil Butte, the mudstones of this unit represent a
pro-delta deposit. Only a few kilometers north of Fossil
Butte, Fossil Lake was replaced by a fluvial flood plain during
this time.

Lower unit time ended with a hypersaline lake where
dolomite precipitated from a shallow body of water and was
clear and free of siliciclastic input. The lake again freshened for
a short period of time before drying up again in one last
gasp before the onset of a dramatic change marking the
beginning of middle unit time.

Cushman (this volume) concludes that the climate
during lower unit time was warm temperate as indicated by
the palynofloral assemblage.

Middle unit time.Middle unit time was a time of
fascinating events in Fossil Basin. It began with a loud roar as
the climate must have changed abruptly, transcending to a
wet-humid period of high rainfall. The lower oil shale at the
base of the middle unit was deposited as a result of the most
expansive phase of Fossil Lake's history. A mudstone unit
(coaly mudstone) rich in coalified plant material with a 2-3 cm
thick coal at the top can be traced throughout the basin. There
are no roots associated. A thin oil shale known as the lower
oil shale (kerogen-rich laminated-micrite) about 30 cm
thick overlies the coal bed. It contains small
Knightia in abundant numbers along with fossil insects and plant fragments.
This sequence is interpreted as being deposited in a lake
undergoing a rapid transgression. The transgression appears to
have been initiated abruptly during a single episodic event that
transported huge amounts of plant material into the lake,
resulting in the thin coal deposit. The high organic-carbon content
and dense fossil fish concentrations are consistent with a lake
in it's early stages of expansion. Nutrient levels are high
and are inherited from the rich soils recently flooded. The
lower oil shale can be traced throughout Fossil Basin wherever
the middle unit outcrops and provides an excellent marker bed
to the experienced stratigrapher.

Fossil Lake remained very productive throughout
middle unit time resulting in the deposition of the fish beds that
the Green River Formation is so famous for. One of these
beds, known as the lower sandwich bed, has been studied
intensively throughout Fossil Basin at over 70 locations
(Trivino, 1996). The two tuff beds delineating the top and bottom
of this unit allow precise stratigraphic correlations and
spatial analysis studies. Studies conducted of bed thickness,
laminae number, stable isotopes, mineralogy, and paleontology
have provided a detailed recreation of the paleogeography of
Fossil Lake during this time. Most of the inflow areas were
concentrated in the southern half of the lake; however a
significant inflow point was present in the northeastern part of the
lake as is indicated by a dramatic thickening of laminae in
the vicinity. The laminae thickening is thought to be a result of
a high rate of calcite precipitation in this area. In addition,
a fluvial channel filled with conglomerate replaces most of
the middle unit a few kilometers east of the thick laminae anomaly.
Buchheim and Biaggi (1988) and Buchheim (1994b)
concluded that the laminae of this unit are not true varves because
laminae number between the two tuff beds increases from
about 1100 to 1600 from lake center to margin. Deposition of
a greater number of laminae near the lake-margins
occurred where calcium-rich inflow water first mixed with the
bicarbonate-rich (alkaline) lake water (Buchheim, 1994a).

Similar results were obtained from a
paleogeographic study (Buchheim, 1993) of the "18-inch layer". This unit
has been extensively excavated for it's well-preserved fossil
fish fauna since the late 1800's. Grande and Buchheim
(1994) provide a detailed analysis of the lateral variation of
paleontological and sedimentological characteristics of this unit.
The 18 inch layer (as a kerogen-rich laminated micrite
containing abundant fossil fishes) is spatially limited to only about a
hundred kilometer area from about the Fossil Butte National
Monument Visitor's center on the north to about 10 kilometers
south; and from about the middle of R119W to the middle of R118W.
About 60% of this area has been eroded out, leaving only
about 40 square kilometers of area where this incredible
paleontologic resource is still preserved. The 18 inch layer only
crops out over a linear distance of about 30 kilometers. Of this
only about 10 kilometers is accessible to major paleontologic
excavation activities, because of steep slopes over most of it
that require excessive overburden removal before collection
can begin. These are rough estimates, but it is clear that the
preservation of this unique paleontological heritage is at risk.
Of the 10 accessible kilometers, up to half may be under
private or state lease to commercial quarry operators! The so
called "split fish" layers at more marginal geographical localities
is not within the 18 inch layer, but within the lower
sandwich beds. The sandwich beds extend over a 40 by 20
kilometer square area centered around location 1306, about 6
kilometers south of Fossil Butte National Monument.

The glory period of Fossil Lake finally faded with
the temporary onset of arrid conditions leading to nearly
total dessication of the lake during middle dolomicrite bed time.
Fossil Lake during this period was clear with little
siliciclastic input. Although this bed is dolomicrite at the center of
the
basin, it grades laterally into calcimicrite. This
observation suggests that fresher conditions exited at the margins of
the lake, a trend that dominates the lake system throughout
it's history. This relationship suggests that the dolomite was
precipitated either directly from the water column or
formed syndepositionally (Buchheim, 1994a), rather than
generated on marginal mudflats. The ostracodal dolomicrite is a
prominent marker bed in the southern half of the basin, where
it forms a hard, dense and blocky unit that leaves "bricks"
of gray limestone scattered over an otherwise soft weathered slope.

Fossil Lake did not experience a major expansion
again until k-spar tuff time (from the top of the ostracodal
dolostone to one meter above the k-spar tuff: a major marker bed that
is composed of authigenic potassium feldspar), but
remained brackish to saline. Abundant fish fossils are presently
being quarried from this unit at some the basin center quarries.
Along with fossil fish occur abundant
Goniobasis gastropods. The significance of this relationship is not well understood
because gastropods generally signal very shallow water and well
oxygenated conditions (Surdam and Stanley, 1979).
Shallow water conditions are indicated by the nearly basin-wide
distribution of kerogen-poor laminated-micrite, a facies
deposited in shallower near-shore water (Buchheim, 1994b) along
with the occurrence of gastropods and abundant burrow traces.
The potassium feldspar mineralogy of the k-spar tuff suggests
hyper saline conditions during this period. Authigenic feldspar
is produced by the reaction of hypersaline-alkaline water
(rich in potassium) with volcanic ash (Sheppard and Gude,
1968, 1969; Surdam and Stanley, 1979). However, the
relationship of feldspar and zeolite mineralogy to salinity is not as clean
as expected. Although authigenic potassium feldspar tuff
beds (rather than zeolite or clay beds) nearly always occur in
dolomite sequences, they occasionally occur in calcitic
sequences as well (Buchheim ,1998). The calcite mineralogy of
the laminated micrite, occurrence of gastropods, and
abundant fossil fishes suggests the lake was fresh to saline, but not
hyper saline.

Cushman (this volume) suggests that the abundance
of hardwood, riparian and conifer taxa provide a picture of
moist lowlands and flood plains around Fossil Lake with
upland forests on the surrounding ridges and mountains. Pine
and other upland taxa grew in the highlands surrounded the lake.
The palynofloral assemblages of the lower and middle
units and the lower part of the upper unit indicate that a
mixed mesophytic forest grew near Fossil Lake. The climate
during middle unit time was probably more subtropical than
during lower unit time.

Upper Unit Time.Fossil Lake developed into a
large, but shallow, hyper saline lake during upper unit time,
generally devoid of fossils in central basin areas (Buchheim, 1994a).
Salt casts of sodium carbonate minerals are abundant
beginning about four meters above the k-spar tuff, indicating
hypersalinity. A series of 3-4 oil shales (kerogen-rich
laminated-micrite) within the lower half of the upper unit
provide marker beds that can be traced throughout the basin.
The lenticular nature of the laminae suggest these were
deposited in shallow water.
<
Did fish entirely disappear from Fossil Lake during
upper unit time? Loewen and Buchheim (1997) recently
reported the first occurrence of fossil fish including very abundant
numbers of Priscacara hypsacantha (perch) and
Lepisosteus (gar) in the upper unit at a near-shore locality south of Elk
Mountain in the southwest part of Fossil Butte (see Loewen
and Buchheim, this volume, for further details). Their
preliminary study indicated that a relatively fresher-water apron
existed on the margins of Fossil Lake. In addition a lateral
trend (lake-center to margin) in mineralogy (dolomite to
calcite), oxygen stable isotopes (heavy to light), petrology
(kerogen-rich laminated-micrite, highly disrupted by salt casts to
laminated calcimicrite to bioturbated calcimicrite) all provide
collaborating evidence of a salinity gradient in Fossil Lake
during upper unit time.

About 16 meters above the k-spar tuff in the upper unit
a four meter thick sequence of chert nodules and bedded
chert occurs in a dolomicrite (chert horizon). The chert
horizon can be correlated throughout Fossil Basin and provides
an excellent marker unit. Lake water pH probably exceeded
10 during this time driving most silicates into solution. The
dissolved silica precipitated out as silica gel and eventually
converted to chert nodules or bedded chert (Buchheim, 1994b).

Abundant fossil birds of the genus
Presbyornis frequented the shorelines of Fossil Lake during upper unit time
(Leggitt and Buchheim, 1997), as indicated by a mass mortality
bird bed that occurs in the vicinity of Warfield Creek about
40 meters above the k-spar tuff. Egg shell fragments,
gastropods and ostracods are commonly associated. The reader is
referred to Leggitt and Buchheim (this volume) for further
details.

Volcanism increased significantly during upper unit
time as indicated by a higher percentage of silicate minerals in
the carbonate rocks (see XRD mineralogy in Figure 1). Tuff
beds tend to be thicker as well. The k-spar tuff, that marks
the lower boundary of the upper unit, averages 13 cm and
increases to a maximum of 25 cm in the northwest corner of
the basin at the head of Watercress Canyon. Most of the tuff
beds in the underlying units are thinner (1-3 cm) and the
carbonates contain generally less than 10% silicate minerals.
The local thickening of the k-spar tuff suggests that a local
volcanic sources existed nearby. If the source was hundreds of
kilometers away, the ash would be more evenly distributed
across the basin.

Pollen collected and studied by Cushman (this
volume) suggest that the climate gradually cooled during upper
unit time, but may have returned to a warmer, subtropical
climate during its final phase. This conclusion is supportedg by
the occurrence of Reevesia (a tropical to subtropical element)
at the top of the upper unit (Cushman, this volume) and is
corroborated by the presence of about a meter of
laminated micrite containing fossil fish. This essentially marked
the end of Fossil Lake's history. Subsidence within the basin
apparently slowed and the fluvial Wasatch Formation the lake.
A few limestones in the Wasatch Formation above the
upper unit indicate that a few short-lived ponds or less
extensive shallow lakes attempted a come back.

We have now completed our walk through time. Our
view is still somewhat hazy, but as research continues in Fossil
Basin it will sharpen and maybe even change. New discoveries
await the motivated researcher; new fossils, new views of
processes occurring in Fossil Lake. This walk will be made again...
and with each walk we will see Fossil Lake with a clearer
perspective!

. 1994a. Eocene Fossil Lake: a history of fluctuating
salinity, p. 239-247. In R. Renaut, and W. Last (eds.),
Sedimentology and geochemistry of modern and ancient saline lakes.
SEPM Special Publication, 50.

. 1994b. Paleoenvironments, lithofacies and varves of
the Fossil Butte member of the Eocene Green River
Formation, Southwestern Wyoming. Contributions to Geology, 30(1):3-14.

Mcgrew, P. O. , and M. Casilliano. 1975. The Geological History
of Fossil Butte National Monument and Fossil Basin: National
Park Service Occasional Paper Number 3. National Park
Service Occasional Paper Number 3, 37 p.